Multi-fluorescent lamp fixtures usually have one or two pairs of lamps with each pair of lamps being ignited and operated by a series-type ballast.
The fluorescent lamps are usually elongate glass envelopes filled with some inert gas such as argon, krypton, or a mixture of inert gases and may have several drops of mercury in the envelope. The inside surface of the glass envelope is coated with phosphors which are capable of fluorescing when activated by ultraviolet radiation. This radiation is produced when the gas within the envelope is ionized. These fluorescent lamps have a negative resistance characteristic and require a higher voltage to start the arc or discharge than is required to maintain the discharge. When a voltage high enough is applied across a fluorescent lamp, the discharge occurs and the impedance of the lamp substantially reduces. As is well known, it is necessary to include in the lamp's circuit a current limiting impedance to prevent the current drawn by the lamp increasing to too high a value.
Also, as is well known, fluorescent lamps usually operate from a source of alternating current supply, and have the ballast in the circuitry for starting and lighting the lamps. The ballast serves as a step-up transformer to raise line voltage to values which will ignite the lamps. The ballast usually has secondary coils loosely coupled with a primary coil so that on open circuit the greatest portion of the transformer step-up voltage is available for ignition without loading the line, but upon ignition, when current flows in the secondary coils, high leakage reactance occurs and provides the impedance necessary to limit the current flow through the lamps while providing a lower operating voltage. Preferably the ballast operates at a high power factor which may be somewhat lagging. Various arrangements are employed to correct for power factor since the winding inductance causes lagging current.
One major disadvantage with multi-lamp fixtures is that when one of a pair of lamps fails, the remaining lamp of the pair cannot be re-ignited so that both lamps then remain unlit. This is due to the ballast circuit being unbalanced by the failure of the one lamp and then not having sufficient power available to re-ignite the other lamp. Not only does this limit the length of the life of the pair of lamps to that of the shorter life of the two, but also due to practical expediency, necessitates both lamps being replaced at the same time so that a lamp with some useful life remaining is discarded.
Another disadvantage is that when one lamp of a pair has failed, the other will start flickering if ambient temperature is 60.degree. F. or above as it tries to discharge, and this creates excess loading on the ballast tending to shorten the useful life thereof.